Adolescent participants will enter the Induction phase and receive icotrokinra daily, orally. At Week I-12 all participants will be evaluated for clinical response and will enter the Maintenance phase.

Adolescent participants who are in clinical response to icotrokinra will enter the Maintenance phase at Week M-0 and continue to receive icotrokinra daily, orally up to Week M-40. Participants who are nonresponders to icotrokinra will also enter the Maintenance phase to receive icotrokinra daily. After completion of the Maintenance phase through Week M-40, eligible participants can participate in a LTE.

Placebo tablet will be administered orally.

Percentage of adult participants with endoscopic improvement at Week I-12 will be reported. Endoscopic improvement is defined as an endoscopy subscore of 0 or 1.

Percentage of adult participants in symptomatic remission at Week I-12 will be reported. Symptomatic remission is defined as a stool frequency subscore of 0 or 1 and a rectal bleeding subscore of 0.

Percentage of adult participants with IBDQ remission at Week I-12 will be reported. IBDQ remission is defined as an IBDQ total score of >= 170. The IBDQ is a validated, 32-item, self-reported questionnaire for participants with IBD that will be used to evaluate the disease-specific health related quality of life (HRQoL) across 4 dimensional scores: bowel symptoms (loose stools, abdominal pain), systemic symptoms (fatigue, altered sleep pattern), social function (work attendance, need to cancel social events), and emotional function (anger, depression, irritability). Scores range from 32 to 224, with higher scores indicating better outcomes.

Percentage of adult participants with HEMI at Week I-12 will be reported. HEMI is defined as achieving a combination of histologic improvement and endoscopic improvement. Histologic improvement is defined as neutrophil infiltration in less than (<) 5% of crypts, no crypt destruction, and no erosions, ulcerations, or granulation tissue according to the Geboes grading system. Endoscopic improvement is defined as an endoscopy sub-score of 0 or 1.

Percentage of adult participants with fatigue response at Week I-12 will be reported. Fatigue response is defined as a >= 7-point reduction in the patient-reported outcomes measurement information system (PROMIS)-fatigue Short Form (SF)-7a total score from baseline. The PROMIS fatigue SF-7a contains 7 items evaluating fatigue-related symptoms (that is, tiredness, exhaustion, mental tiredness, and lack of energy) and associated impacts on daily activities (that is, activity limitations related to work, self-care, and exercise).

Percentage of adult participants in symptomatic remission at Week I-4 will be reported. Symptomatic remission is defined as a stool frequency subscore of 0 or 1 and a rectal bleeding subscore of 0.

Percentage of adult participants in endoscopic remission at Week I-12 will be reported. Endoscopic remission is defined as an endoscopy subscore of 0.

Percentage of adult participants with no bowel urgency at Week I-12 will be reported using the ulcerative colitis patient-reported outcomes signs and symptoms (UC-PRO/SS). The UC-PRO/SS is a 9-item measure that was developed to standardize the quantification of gastrointestinal (GI) signs and symptoms of UC through direct report from participant ratings. The module includes both bowel signs and symptoms and abdominal symptoms, with a recall period of 24 hours.

Percentage of adult participants with no abdominal pain at Week I-12 will be reported using the UC-PRO/SS. The UC-PRO/SS is a 9-item measure that was developed to standardize the quantification of GI signs and symptoms of UC through direct report from participant ratings. The module includes both bowel signs and symptoms and abdominal symptoms, with a recall period of 24 hours.

Percentage of adult participants with no bowel incontinence at Week I-12 will be reported using the UC-PRO/SS. The UC-PRO/SS is a 9-item measure that was developed to standardize the quantification of GI signs and symptoms of UC through direct report from participant ratings. The module includes both bowel signs and symptoms and abdominal symptoms, with a recall period of 24 hours.

Percentage of adult participants with AEs and SAEs will be reported. An AE is any untoward medical occurrence in a clinical study participant administered a pharmaceutical (investigational or non-investigational) product that does not necessarily have a causal relationship with the intervention. A SAE is any untoward medical occurrence that at any dose results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, is a suspected transmission of any infectious agent via a medicinal product, is medically important.

Percentage of adult participants with endoscopic improvement at Week M-40 will be reported. Endoscopic improvement is defined as an endoscopy subscore of 0 or 1.

Percentage of adult participants in symptomatic remission at Week M-40 will be reported. Symptomatic remission is defined as a stool frequency subscore of 0 or 1 and a rectal bleeding subscore of 0.

Percentage of adult participants with 90-day corticosteroid-free clinical remission at Week M-40 will be reported. 90-day corticosteroid-free clinical remission is defined as the clinical remission at the visit and not receiving corticosteroids for 90 days prior to the visit.

Percentage of adult participants with HEMI at Week M-40 will be reported. HEMI is defined as achieving a combination of histologic improvement and endoscopic improvement. Histologic improvement is defined as neutrophil infiltration in < 5% of crypts, no crypt destruction, and no erosions, ulcerations, or granulation tissue according to the Geboes grading system. Endoscopic improvement is defined as an endoscopy sub-score of 0 or 1.

Percentage of adult participants with IBDQ remission at Week M-40 will be reported. IBDQ remission is defined as an IBDQ total score of >= 170. The IBDQ is a validated, 32-item, self-reported questionnaire for participants with IBD that will be used to evaluate the disease-specific HRQoL across 4 dimensional scores: bowel symptoms (loose stools, abdominal pain), systemic symptoms (fatigue, altered sleep pattern), social function (work attendance, need to cancel social events), and emotional function (anger, depression, irritability). Scores range from 32 to 224, with higher scores indicating better outcomes.

Percentage of adult participants in endoscopic remission at Week M-40 will be reported. Endoscopic remission is defined as an endoscopy subscore of 0.

Percentage of adult participants in clinical remission at Week M-40 among the participants who had achieved clinical remission at maintenance baseline will be reported. Clinical remission is defined as stool frequency subscore of 0 or 1, a rectal bleeding subscore of 0, and an endoscopy subscore of 0 or 1.

Percentage of adult participants in histologic-endoscopic mucosal remission at Week M-40 will be reported. Histologic-endoscopic mucosal remission is defined as achieving a combination of histologic remission and endoscopic remission. Histologic remission is defined as the absence of neutrophils from the mucosa (both lamina propria and epithelium), no crypt destruction, and no erosions, ulcerations or granulation tissue according to the Geboes grading system. Endoscopic remission is defined as an endoscopy subscore of 0.

Percentage of adult participants with fatigue response at Week M-40 will be reported. Fatigue response is defined as a >= 7-point reduction in the PROMIS-fatigue SF-7a total score from baseline. The PROMIS fatigue SF-7a contains 7 items evaluating fatigue-related symptoms (that is, tiredness, exhaustion, mental tiredness, and lack of energy) and associated impacts on daily activities (that is, activity limitations related to work, self-care, and exercise).

Percentage of adult participants with disease clearance at Week M-40 will be reported. Disease clearance is defined as a composite of symptomatic remission and histologic-endoscopic mucosal remission. Symptomatic remission is defined as a stool frequency subscore of 0 or 1 and a rectal bleeding subscore of 0. Histologic-endoscopic mucosal remission is defined as achieving a combination of histologic remission and endoscopic remission.

Percentage of adult participants with no bowel urgency at Week M-40 will be reported using the UC-PRO/SS. The UC-PRO/SS is a 9-item measure that was developed to standardize the quantification of GI signs and symptoms of UC through direct report from participant ratings. The module includes both bowel signs and symptoms and abdominal symptoms, with a recall period of 24 hours.

Percentage of adult participants with no abdominal pain at Week M-40 will be reported using the UC-PRO/SS. The UC-PRO/SS is a 9-item measure that was developed to standardize the quantification of GI signs and symptoms of UC through direct report from participant ratings. The module includes both bowel signs and symptoms and abdominal symptoms, with a recall period of 24 hours.

Percentage of adult participants with no bowel incontinence at Week M-40 will be reported using the UC-PRO/SS. The UC-PRO/SS is a 9-item measure that was developed to standardize the quantification of GI signs and symptoms of UC through direct report from participant ratings. The module includes both bowel signs and symptoms and abdominal symptoms, with a recall period of 24 hours.

Percentage of adult participants with AEs and SAEs will be reported. An AE is any untoward medical occurrence in a clinical study participant administered a pharmaceutical (investigational or non-investigational) product that does not necessarily have a causal relationship with the intervention. A SAE is any untoward medical occurrence that at any dose results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, is a suspected transmission of any infectious agent via a medicinal product, and is medically important.

Percentage of adolescent participants in clinical response at Week I-12 will be reported. Clinical response is defined as a decrease from baseline in the modified Mayo score by greater than or equal to (>=) 30 percent (%) and >=2 points, with either a >=1-point decrease from baseline in the rectal bleeding subscore or a rectal bleeding subscore of 0 or 1.

Percentage of adolescent participants in clinical remission at Week I-12 will be reported. Clinical remission is defined as stool frequency subscore of 0 or 1, a rectal bleeding subscore of 0, and an endoscopy subscore of 0 or 1.

Percentage of adolescent participants with HEMI at Week I-12 will be reported. HEMI is defined as achieving a combination of histologic improvement and endoscopic improvement. Histologic improvement is defined as neutrophil infiltration in less than (<) 5% of crypts, no crypt destruction, and no erosions, ulcerations, or granulation tissue according to the Geboes grading system. Endoscopic improvement is defined as an endoscopy sub-score of 0 or 1.

Percentage of adolescent participants with endoscopic improvement at Week I-12 will be reported. Endoscopic improvement is defined as an endoscopy subscore of 0 or 1.

Percentage of adolescent participants in symptomatic remission at Week I-12 will be reported. Symptomatic remission is defined as a stool frequency subscore of 0 or 1 and a rectal bleeding subscore of 0.

Percentage of adolescent participants in endoscopic remission at Week I-12 will be reported. Endoscopic remission is defined as an endoscopy subscore of 0.

Percentage of adolescent participants in PUCAI remission at Week I-12 will be reported. PUCAI remission is defined as a PUCAI score <10. The PUCAI is a non-invasive instrument for measuring disease activity in adolescents with UC. Disease activity is determined as a score based on evaluation of abdominal pain, rectal bleeding, stool consistency, number of stools, nocturnal bowel movement, and activity level. The PUCAI score ranges from 0 to 85.

Percentage of adolescent participants with AEs and SAEs will be reported. An AE is any untoward medical occurrence in a clinical study participant administered a pharmaceutical (investigational or non-investigational) product that does not necessarily have a causal relationship with the intervention. A SAE is any untoward medical occurrence that at any dose results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, is a suspected transmission of any infectious agent via a medicinal product, and is medically important.

Percentage of adolescent participants with endoscopic improvement at Week M-40 will be reported. Endoscopic improvement is defined as an endoscopy subscore of 0 or 1.

Percentage of adolescent participants in symptomatic remission at Week M-40 will be reported. Symptomatic remission is defined as a stool frequency subscore of 0 or 1 and a rectal bleeding subscore of 0.

Percentage of adolescent participants in 90-day corticosteroid-free clinical remission at Week M-40 will be reported. 90-day corticosteroid-free clinical remission is defined as the clinical remission at the visit and not receiving corticosteroids for 90 days prior to the visit.

Percentage of adolescent participants with HEMI at Week M-40 will be reported. HEMI is defined as achieving a combination of histologic improvement and endoscopic improvement. Histologic improvement is defined as neutrophil infiltration in < 5% of crypts, no crypt destruction, and no erosions, ulcerations, or granulation tissue according to the Geboes grading system. Endoscopic improvement is defined as an endoscopy sub-score of 0 or 1.

Percentage of adolescent participants with histologic-endoscopic mucosal remission at Week M-40 will be reported. Histologic-endoscopic mucosal remission is defined as achieving a combination of histologic remission and endoscopic remission. Histologic remission is defined as the absence of neutrophils from the mucosa (both lamina propria and epithelium), no crypt destruction, and no erosions, ulcerations or granulation tissue according to the Geboes grading system. Endoscopic remission is defined as an endoscopy subscore of 0.

Percentage of adolescent participants with endoscopic remission at Week M-40 will be reported. Endoscopic remission is defined as an endoscopy subscore of 0.

Percentage of adolescent participants in clinical remission at Week M-40 among the participants who had achieved clinical remission at maintenance baseline will be reported. Clinical remission is defined as stool frequency subscore of 0 or 1, a rectal bleeding subscore of 0, and an endoscopy subscore of 0 or 1.

Percentage of adolescent participants in clinical response at Week M-40 among the participants who had achieved clinical response at maintenance baseline will be reported. Clinical response is defined as a decrease from baseline in the modified Mayo score by >= 30 % and >=2 points, with either a >=1-point decrease from baseline in the rectal bleeding subscore or a rectal bleeding subscore of 0 or 1.

Percentage of adolescent participants in PUCAI remission at Week M-40 will be reported. PUCAI remission is defined as a PUCAI score <10. The PUCAI is a non-invasive instrument for measuring disease activity in adolescents with UC. Disease activity is determined as a score based on evaluation of abdominal pain, rectal bleeding, stool consistency, number of stools, nocturnal bowel movement, and activity level. The PUCAI score ranges from 0 to 85.

Percentage of adolescent participants with AEs and SAEs will be reported. An AE is any untoward medical occurrence in a clinical study participant administered a pharmaceutical (investigational or non-investigational) product that does not necessarily have a causal relationship with the intervention. A SAE is any untoward medical occurrence that at any dose results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, is a suspected transmission of any infectious agent via a medicinal product, and is medically important.